Isolation and Characterization of rpoS from a Pathogenic Bacterium, Vibrio vulnificus : Role of σ S in Survival of Exponential-Phase Cells under Oxidative Stress

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Abstract
A gene homologous to rpoS was cloned from a fatal human pathogen, Vibrio vulnificus . The functional role of rpoS in V. vulnificus was accessed by using an rpoS knockout mutant strain. This mutant was impaired in terms of the ability to survive under oxidative stress, nutrient starvation, UV irradiation, or acidic conditions. The increased susceptibility of the V. vulnificus mutant in the exponential phase to H 2 O 2 was attributed to the reduced activity of hydroperoxidase I (HPI). Although σ S synthesis was induced and HPI activity reached the maximal level in the stationary phase, the mutant in the stationary phase showed the same susceptibility to H 2 O 2 as the wild-type strain in the stationary phase. In addition, HPII activity, which is known to be controlled by σ S in Escherichia coli , was not detectable in V. vulnificus strains under the conditions tested. The mutant in the exponential phase complemented with multiple copies of either the rpoS or katG gene of V. vulnificus recovered both resistance to H 2 O 2 and HPI activity compared with the control strain. Expression of the katG gene encoding HPI in V. vulnificus was monitored by using a katG :: luxAB transcriptional fusion. The expression of this gene was significantly reduced by deletion of σ S in both the early exponential and late stationary phases. Thus, σ S is necessary for increased synthesis and activity of HPI, and σ S is required for exponentially growing V. vulnificus to develop the ability to survive in the presence of H 2 O 2 .